Levi King, 27, pleaded guilty last month to killing Michell Conrad, 35, her husband Brian Conrad, 31, and her 14-year-old son, Zach Doan. Michell Conrad was six months pregnant.
The Lubbock County jury had to answer two questions in order to give King the death penalty: Was he a continuing threat to society? Did mitigating circumstances exist that warranted mercy?
Jurors were only able to answer yes to the first and were unable to answer the second, making the life sentence automatic.
"We're gratified the system worked like we wanted it to," said defense attorney Maxwell Peck III.
Prosecutor Lynn Switzer declined to comment on the sentence.
During nearly a month of testimony in the sentencing phase of the trial, prosecutors presented photographs showing the three family members' bullet-ridden bodies lying in or near their beds and played audio from the 911 call made by the sole survivor, Robin Doan, who was 10 years old at the time of the killings.
The girl called authorities after King left the family's farmhouse near Pampa and could be heard sobbing as she told the dispatcher: "I want my mommy."
The now 14-year-old testified that she awoke to her mother's screams the night of Sept. 30, 2005, and heard the killer go from one room to another, shooting her family members.
After the verdict, Robin Doan told King she forgives him. She also said she felt guilty about not being able to save her family, The Pampa News reported. "Why did God pick me to be here and not them?" she said.
Peck had pleaded for King's life, asking jurors to resist vengeance.
The court-appointed attorney had told jurors King grew up in poverty in Missouri and was emotionally neglected by both parents, who used drugs regularly. King had earlier told the judge that he'd been diagnosed with bipolar disorder and paranoid schizophrenia; he also has been told he was psychotic. Peck said King had been suicidal.
King also pleaded guilty last year in the shooting deaths of Orlie McCool, 70, and his 47-year-old daughter-in-law, Dawn McCool. Their bodies were found by a relative in a rural Pineville, Mo., home on Sept. 30, 2005 _ the same day authorities in the Texas Panhandle discovered the bodies in the Conrads' home. Authorities said King drove Orlie McCool's pickup truck from Missouri to Texas.
King was caught the same night trying to re-enter the United States at the Mexican border in El Paso, Texas.
In a plea agreement with Missouri prosecutors, King was sentenced to consecutive life terms without parole in the McCools' deaths.